The function of an electronic package is to permit handling and installation of the electronic components without damage, and to protect the components from environmental factors such as moisture and corrosive agents. It must also provide a pathway to the environment for the electrical current controlled by the device and for heat generated by components within the package. The package of an electronic device or module is basically any part of the module except the silicon die. The package includes, for example, the die attach solder, heat spreader, any electrical insulator such as BeO, heat sink, bond wires, interconnections and connectors, and the housing. The cost of the packaging is usually greater than the cost of the silicon devices within the package and the quality of the package influences the performance and reliability of the device or module as well as the price.
The extraction of heat generated by components within the package is necessary to prevent those components from reaching a temperature where their reliability is adversely affected. This is a particularly important problem for modules which contain power transistors which can dissipate tens of watts of energy during their normal operation and require low resistance electrical contacts to minimize parasitic power losses. The electrical resistance of the packaging must be much lower than the on resistance of the power transistor die so that the voltage drop of the packaged device is minimal and determined primarily by the die on-resistance. The thermal expansion coefficients of the various materials making up the package must be well matched to avoid excessive mechanical stresses which could rupture the electrical and thermal interfaces and cause failure.
High power solid state devices, such as transistors, diodes, and silicon controlled rectifiers, commonly utilize the so-called vertical structure where the current passes vertically through the die from top side contact to the bottom side die mount (FIG. 1). The mechanical contact made by the die mount to the die must accommodate the mechanical stresses that develop when the die and the die mount are at a temperature different from the die attach solder solidification temperature and when the temperature of the die and die mount thermally cycle during normal operation. The stress relief function is a particularly demanding one in that the solder die attach layer plastically deforms to limit the stress transmitted to the die. This plastic deformation introduces structural defects, point defects and dislocations, which will ultimately lead to mechanical fatigue and failure of the die attach layer. Mechanical fatigue of the die attach solder can be reduced by using a harder, less pliable solder, but with the penalty of increasing the mechanical stresses applied to the die. In worse case situations, the stresses can become large enough to fracture the die, resulting in failure of the device and usually the circuit in which it is embedded. This is particularly true for large power transistor die. Industry rule of thumb is that the largest die that can be directly soldered to a solid copper heat sink that is about 0.63 to 0.76 cm (0.25 to 0.3 inch) square. Many man years of research and development have been invested in improving the reliability of power transistor packages, the die attach solder in particular, to achieve a transistor operating life in excess of the useful life of the circuits in which they are used. However, the package will ultimately fail because the principal failure mode, solder fatigue, is built into the structure of the package.
In a study of solder fatigue by Vaynman and McKeown, "Energy-Based Methodology for the Fatigue Life Prediction of Solder Materials," IEEE Transactions On Components, Hybrids, and Manufacturing Technology, Vol. 16, No. 3, pp. 317, 1993, the number of shear stress-strain cycles a solder joint can experience before failing is correlated with the damage to the solder caused by the deformation. Shear stress-strain cycles are a natural consequence of the temperature cycling a component experiences in normal operation. The damage function is defined as the product of the shear stress and the shear strain, i.e., the work performed on the solder in the plastic deformation cycle. With repeated cycling, the damage accumulates and the joint fails. Reducing shear strain reduces solder damage, and extends solder joint life. The shear strain is reduced by reducing the shear stress.
To improve die attach reliability and increase operating life, it is desirable to decouple the electrical and thermal contact function from the stress accommodation function. This would make a wider parameter space available for packaging power electronic systems. One could envision several ways to achieve this decoupling--use a liquid metal contact between die and heat sink, with the risk that the die could be dislodged by shock or vibration, or by making a dry pressure contact between die and heat sink, but dry contacts have higher electrical and thermal resistance.
The use of copper wire bundles in place of solid copper heat sinks to relieve stress on power transistor die was first reported by H. H. Glascock and H. F. Webster, "Structured Copper: A Pliable High Conductance Material for Bonding to Silicon Power Devices", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-6, No. 4, pp. 460-466, 1983. J. F. Burgess, R. O. Carlson, H. H. Glascock, II, C. A. Neugebauer, and H. F. Webster, "Solder Fatigue in Power Packages", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-7, No. 4, pp. 405, 1984, also discloses packaging arrangements of interest. Glascock and Webster were concerned with packaging the very large (several square inches in area) power devices used in electric power transmission equipment. They prepared their wire bundles by winding skeins of copper wire, inserting the skein in a copper tube, and drawing the tube to compress the wires tightly together. Their test results clearly showed the effectiveness of this concept in minimizing the stresses on very large die and significantly extending, their reliable operating life. However, the complexity and time consuming nature of the process used to prepare the copper wire bundles evidently intimidated others in the field because there have been no other reports in the literature exploring this novel concept in the eleven years since the publication of their results.
The present invention overcomes many of the disadvantages of the prior art.